Method and apparatus for inductively charging a filter of combined metal and dielectric material for collecting normally charged air borne particles

ABSTRACT

Monocharging a filter consisting of electroconductive and nonconductive collecting surfaces with an inductive field supplied from a low frequency pulsating direct current voltage to attract and collect normally charged particles. This inductive filter charging field permits the use of a metal plate or grid filter or a monocharged filter of highly insulative materials including paper, plastics such as fiber glass, open cell foam or plastic screen. A grid conductor must be employed to charge these nonconductive dielectric materials. Metal plates, wire screens and grids, metal shavings, metal wools inductively charged distribute the monocharge of low voltage low frequency over their own surfaces as well as the insulating or dielectric portions of the filter. This monocharged filter requires no ground or return circuit and merely produces a slight shock if touched but is not harmful.

United States Patent. 1 3,581,462

721 lnventor WilIiamWStum 3,053,028 9/1962 Kayko 55/103 27 Locus! Drive.B i geville, Pa. 15017 3,173,772 3/1965 Gelfand 55/105 [21] A pplNo.786,217 3,326,401 6/1967 DeLong 215/56X [22] Filed Dec-23,1963 3,406,66910/1968 Edwards l23/ll9(B) [45] Pal J 1,1971 3,446,906 /1969 Zulauf55/524X FOREIGN PATENTS 248,429 /1963 Australia 55/105 [54] METHOD ANDAPPARATUS FOR INDUCTIVELY CHARGING A FILTER OF COMBINED METAL ANDDIELECTRIC MATERIAL FOR COLLECTING NORMALLY CHARGED AIR BORNE PARTICLESPrimary ExaminerDennis E. Talbert, Jr. Attorney-Carothers and CarothersABSTRACT: Monocharging a filter consisting of electroconductive andnonconductive collecting surfaces with an inductive field supplied froma low frequency pulsating direct current voltage to attract and collectnormally charged particles. This inductive filter charging field permitsthe use of a metal plate or grid filter or a monocharged filter ofhighly insulative materials including paper, plastics such as fiberglass, open cell foam or plastic screen. A grid conductor must beemployed to charge these nonconductive dielectric materials. Metalplates, wire screens and grids, metal shavings, metal wools inductivelycharged distribute the monocharge of low voltage low frequency overtheir own surfaces as well as the insulating or dielectric portions ofthe filter. This monocharged filter requires no ground or return circuitand merely produces a slight shock if touched but is not harmful.

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I METHOD AND APPARATUS FOR INDUCTIVELY CHARGING A FILTER OF COMBINEDMETAL AND DIELECTRIC MATERIAL FOR COLLECTING NORMALLY CHARGED AIR HORNEPARTICLES BACKGROUND OF THE INVENTION V US. Pat. No. 3,040,497 of Class55, Subclass 112, is directed to Electrostatic Gas Filters wherein anelectrostatic charge is applied to each of two filters of glass floss orfiber in combination with vertical rod grids to increase theeffectiveness of filtering out dust passing therethrough. The twofilters are insulated from ground and extend across the duct opening butare spaced from each other, being placed 2 inches in tandem for the flowto pass therethrough. A high voltage high frequency AC supply is chargedon one filter and a 30,000 volt pulsating DC voltage charge is chargedto the other filter. These two charges on the two filters create anionizer in one and a collector in the other and electrostatic chargessurround the grids of each filter and they combine to charge the areabetween the filters to form an ionizer and a dense electrostatic barrierto dust particles. In this respect their circuits depend upon the commongrounds of each circuit to provide intere- Iectrical cooperation betweenthe two spaced filters with a definite limitation upon their spacing.

SUMMARY OF THE INVENTION The principal object of this invention is theprovision of method and apparatus of filtering normally charged airborne particles on metallic filters as well as combined metallic anddielectric materials by a novel method and circuit of charging thefilter with an inductive field induced by a low frequency pulsatingdirect current voltage.

Any type of well-known metal filter may be employed with the methodcomprising this invention. A dielectric filter when properly chargedfunctions equally well. A wire mesh screen, grid, or hardware cloth of asheathed wire with or without plastic fibers or mesh will properlyfunction as a collecting medium. They may be readily washed or disposedof by replacing the plastic, glass fibers, paper tissues ro otherdielectric material. The charging grid may then be vacuumed and newdielectric material applied. Any dielectric material can be used for thedielectric filter if the material can be inductively charged. Thus, thedielectric materials are not limited to those examples mentioned aboveor elsewhere in the application.

Another object of this invention is the provision of a air permeablefilter of dielectric material and of metallic material of a combinationof both. Although such filters are old for use without charging, theiruse with a monocharge of low frequency pulsating direct current voltagehaving the characteristic of a spike discharge of narrow width and shortduration provides a highly efficient dust collector. This inductivecharging system is safe for several reasons. The short spike pulse for lmillisecond cannot burn or cause muscles to freeze to the chargedelement. The current capacity is in the nature of l microampere whichcannot be destructive.

The circuit itself is novel and provides a pulse initiating circuitmeans which includes a silicon controlled rectifier operating a parallelor loop circuit characterized as a high voltage low impedance dischargecircuit and including a condenser and autotransformer primary of thespark coil type, which delivers a spike discharge of a very narrow widthand short duration. The condenser charges and fires the transformerthrough a diode to produce a monocharging inductive field on the filter.This output distributed by a metal screen or grid charges paper or fiberglass which filters and collects charged particles such as dust and lintfrom 1 to 200 microns at an efficiency of from 86 percent to 92 percent.

The gate controlled'rectifier is supplied by AC through a half wavediode boosted by a condenser and through a load resistor and ispreferably pulsated by a field effect transistor, all which are includedin the pulse initiating circuit means and is the preferred form ofobtaining low frequency DC pulsations. Another mode of producing gatepulsations is by the use of a neon lamp operated by a pulsing circuit.The SCR feeds the loop circuit including the primary of the spark coiltype autotransformer in series with a condenser which also stops thefiring of the SCR after each spike type voltage discharge from theautotransformer in each circuit.

Other objects and advantages appear hereinafter in the followingdescription and claims.

The accompanying drawings show, for the purpose of exem plificationwithout limiting the invention or the claims thereto, certain practicalembodiments illustrating the principles of this invention wherein:

FIG. I is a plan view of a filter frame of insulating material with asingle 20 mesh metal wire screen.

FIG. 2 is a plan view of a filter frame of insulating material with aplastic screen of approximately 20 or 30 mesh and having a charging wiregrid attached thereto.

FIG. 3 is a sectional view of a filter insulating frame having I0 meshmetal hardware cloth flanked by open cell foam covering the mesh andheld in place by large mesh plastic retaining grids.

FIG. 4 is a perspective view of an ordinary fiber glass filter found inopen market with an inserted rod charging grid.

FIG. 5 is a sectional view of a filter constructed of a metal woolenclosed in a plastic screen secured to a frame of insulating material.

FIG. 6 is a perspective view of a filter constructed of parallel platesaligned in a frame of insulated material and constructed of metalhardware cloth having paper tissue attached to each side to be alignedin the airstream.

FIG. 7 is a circuit diagram of a preferred form for generating a lowfrequency pulsating direct current voltage to inductively charge afilter of combined metal and dielectric.

FIG. 8 is a modified circuit diagram for generating a low frequencypulsating direct current voltage to inductively charge a filter ofcombined metal and dielectric.

FIG. 9 is a reproduction of the spike waveform that occurs at the filterfor either circuit disclosed.

Referring to FIG. I of the drawings, the insulating frame 1 of thisfilter may be made of wood or plastic of either thermal plastic orthermal setting type. It should have fair nonmoisture absorptioncharacteristics and be capable of supporting parts such as filteringmaterial and have wire or plastic screens fastened thereto. For thispurpose an offset shoulder 2 is formed in the frame opening at thecenter of the frame 1 to receive a filter body such as the twenty meshwire screen 3 or grid shown in place in FIG. I. The edges of this screenor grid fits on the surrounding surfaces 4 which is disposed at rightangles to the shoulder 2 and extend around the frame 1. This completesthis filter 5 of FIG. I. It is nothing but a wire screen in an insulatedframe. The edges of the screen are nailed, cemented or glued to theframe shoulder 2.

This filter 5 is for insertion in the airflow conduit transverse of theflow of air or gas to be cleaned which passes therethrough. A coveredand shielded conductor 6 is connected to this metallic screen 3 and islead off through the frame I at the most convenient place for connectionto the circuit. The control box carrying the circuit of FIg. 7 or 8 issmall and is mounted conveniently on the air duct through which the airtravels and is connected by a plug which may be withdrawn to open thecircuit and remove the filter from the duct.

Referring now to FIG. 2, the frame 1 in this instance is the same asthat illustrated in FIG. 1 and is provided with a plastic screen inplace of a wire screen as illustrated in FIG. 1. This plastic screen isindicated at 7 and may be constructed of a 20 or 30 mesh. This plasticscreen is provided with a metal grid 8 which may be made of either bareor coated wire which is woven or otherwise secured in a pattern to theplastic screen 7. As illustrated in FIG. 2, thedimensions of wire gridis considerably larger than the mesh of the plastic screen 7 but itcovers substantially all of the plastic screen and it may extend to theframe I as shown in FIG. 1. Again the grid may be spaced from the frame1 depending upon the need for the distribution of the induced field onthe plastic screen.

When a grid of this character substantially covers the whole of theplastic screen, thus the plastic screen is provided with the same chargeand will be equally effective in attracting any charged particles thatpass through this filter 9. This filter is also provided with a shieldedand properly insulated conductor 6 for attaching the same by means ofaplug to the box containing the circuit as shown in FIGS. 7 and 8.

The filter structure 10 illustrated in FIG. 3 is provided with the frame11 which may preferably be from 2 or 3 inches thick and the center ofthe frame is provided with a wire of 10 mesh metal hardware cloth screen12 having attached thereto in suitable manner the shielded and insulatedconductor 6 for securing this filter to the circuit. Both of the outerfaces of the screen are covered with a foamed, open cell flexiblepolyester structure 13 held in place by a large open mesh outer grid 14of plastic. This character of open cell foam is very porous. One can seethrough it but it provides a maze of intricate deviations through theopenings created by the open plastic structure. Thus the plastic grids14 hold the open cell foam plastic mesh on each side of the electrodegrid connected to the wire or cable 6.

When the grid 12 and its lead connection 6 is energized it alsoenergizes the open cell plastic 13 together with the plastic outer grids14 that retain the same in the frame 11. This construction makes a veryfine filter even though the majority of the materials of the filter aremade of plastic or some form of dielectric material such as fiber glass,spun wools of different character of plastics.

Referring to FIG. 4, the filter 15 is an ordinary fiber glass filterthat can be purchased in the open market at almost any hardware store.This filter is made of cardboard and is filled with the fiber glass andapplicant can readily change this into a more effective filter bycutting a slit 16 in the cardboard across the top of the filter andinserting therein a grid 17 in the form of a series of copper rods orwires which extend substantially the full depth of the filter and areconnected together at the top by the lead wire 18. The wire 18 connectseach of the rods of the grid 17 and is in turn connected by theinsulated lead 6 which extends to the circuit control box as describedwith reference to each of the foregoing figures.

This fiber glass filter thus becomes a charged filter similar to each ofthe other filter structures and the fiber glass becomes charged and willattract any charged particle passing therethrough. Thus by adding thecharged grid of this invention to a filter that is made of material thatcannot conduct electricity it is nevertheless capable of accepting aninductive field from the circuit disclosed and is effective to increaseits ability to remove the dirt as a filter many times more efficientlythan that without this inductive charge. This type of a filter 15 isreferred to as a throw away type. Thus by retaining the grid 17 inthe'form of the rods one may purchase the filter of fiber glass in apaper container as illustrated and insert the grid 17 and when thefilter is ready to be discarded merely remove the grid 17 wash the sameand insert it in a new filter structure. This could not be possible ifit were not for the inductive field of low frequency pulsating directcurrent im pressed on this insulating material and effective for it tofunction in turn to draw and hold the charged particles passing throughthe filter. This is an improved advancement and is an important objectof this invention.

Referring now to FIG. 5, the filter 20 is provided with an insulatingframe 21 and retains a pillowlike structure covered on both sides withplastic screen 22 of suitable mesh containing aluminum wool 23 or thelike. The aluminum wool 23 is quite light and when stuffed in thepillow-shaped plastic screen mesh 22 it functions as a very good screenthat may be readily cleaned either by a vacuum cleaner or if the dirtbecomes too greasy it may be readily washed.

In all instances where a plastic screen is employed as the only memberor an outer covering for any other material, it may be best to vacuumboth sides of the plastic filter before washing the same. A considerableportion of the dirt may be removed by the vacuum cleaner from the outersurfaces of the filter and those surfaces within the filter if notproperly cleansed by this force of vacuum cleaner, may be readilyremoved by washing with a detergent or other means and then blowing theair through the filter in order to dry the same.

lt is obvious that since the contents 23 within the plastic screen suchas aluminum, copper of iron wool is electrically conductive there is noquestion about the fact that the'whole screen is provided with aninductive field by the attachment of the lead in wire 6 as previouslydescribed.

Referring now to FIG. 6, the filter 24 depicts a filter with parallelplates that would be of the character included in a Cottrell type systemwhere ionization is provided by an ionization grid before the air passesbetween parallel plates which are oppositely polarized.

Solid parallel plates may be employed with the present invention but theplates 25 shown, are made of hardware wire screen of 10 mesh or more.The screen or grid plates 25 are preferably dipped in a readily solubleliquid glue and tissue such as a cleaning or facial tissue paper is thenplaced on each side of the wire mesh as indicated at 26. Each of theplates or screen grids 25 are carried in an insulating slotted frame ortube section 27 of the filter 28 and when the filter 28 is removed formthe system the plates or grids 25 may be withdrawn and washed to removethe paper with the dirt. The grid is then dried and redipped in asoluble glue substance to again retain a layer or two of the facialtissues on each side of the grid and reinserted in the filter and thenceinto the duct in service.

Each of the plates or grids 25 may be connected together in the singlefilter 28 by a single lead 6 to connect the series to the control boxand thereby energize each of the grids with the same inductive field.

Paper may be employed on a filter that requires the air to pass throughthe paper tissue however such a filter restricts the flow of airsomewhat and is not desirable for gravity or low force circulating typeheating systems although it is quite effective in the filter such asillustrated in FIG. 6.

One thing about the use of an inductive field as employed in filters ofthis invention on electroconductive as well as insulated material isthat a moderately low voltage is employed for the filtering charge whichmay be from 3 to 6,000 volts with a current capacity of one tenth of amilliamp which potential is not to ground and produces merely aninductive type of potential. Whether or not a person is grounded theycan receive a shock from the charged filter. However, the filterfunctions without any need for a ground connection and in this respectthe inductive field is charged to the dielectric filter as a monochargevoltage.

Another important object and advantage of this application is thesimplicity of the circuit for effecting a monocharge for inducing afield on conductive as well as nonconductive materials for the purposeof precipitating or drawing out of the air any particles which normallycarry a charge. ln its simplicity, it is also less expensive and notharmful. It can be applied to gravity as well as circulating systems.

Referring to FIG. 7, the circuit is supplied with l 10 alternatingcurrent through the lines L1 and L2, L1 being considered the voltageline whereas L2 is considered to be a ground connection. L1 is connectedto the anode of the diode 30 the cathode of which is connected to oneside of the condenser 31 and also connected by the line 32 to one end ofeach of the resistors 33 and 34. The other end of resistor 33 isconnected to one end of each of the resistors 35 and 36. The resistor 35is connected to the base B2 of the field effect transistor 37. Theemitter E of the field effect transistor 37 is connected to the otherend of the resistance 36 and to one end of the condenser 38. The B1 ofthe field effect transistor 37 is connected to one end of the resistance40 and to the gate G of the silicon controlled rectifier 41 the anode ofwhich is connected to the other end of resistor 34 and to one end of thecondenser 42. The other end of condenser 42 is connected to one end ofthe primary P of the autotap transformer 43.

Line 2 is connected to the other end of each of the condenser 31, thecondenser 38, the resistance 40, the cathode C of the silicon controlleddiode 41 and the other end of the primary winding P of the autotaptransformer 43.

This completes the pulse initiating circuit means which includes a meansto rectify the alternating current together with the field effecttransistor which supplies a pulsating charge to the gate of the siliconcontrolled rectifier 41 which is loaded by the resistance 34. Thecontrolled rectifier supplies a current flow through the chargingcondenser 42 to the primary of the autotap transformer 43 of thedischarge circuit. The condenser 42 also shuts off the SCR and alsocharges the transformer primary to produce a spike type voltagedischarge through the diode 44 of the discharge circuit to the filter orcollection unit as indicated at 45 in FIG. 7. As previously pointed outthis circuit needs no return because it is a monocharge filter producingan inductive field not dependent upon a return circuit.

Thus, the circuit described will produce an inductive charge on thefilter when provided with the proper grid containing a voltage of from 3to 6,000 volts DC but has a very low current capacity. The field effecttransistor provides a pulsation of the direct current voltage in thenature of 60 to 70 cycles per second which is an extremely low frequencyfor a pulsating direct current voltage. This is the principal reason whythis inductive field charging apparatus is ideal for domesticapplication in gravity as well as circulating forced air furnaces. Itwill effectively remove any air borne particlesthat are provided with acharge and could not in any way be serious or harmful to persons whocome in contact with the charged field.

Referring to FIG. 8, wherein a more simplified and alternate circuit isshown connected to the HO volt 60 cycle lines L1 and L2 is the diode 30the cathode of which is connected to one side of the condenser 31 theother side being connected to line 2. The cathode of the diode 30 islikewise connected to one end of the silicon controlled rectifier loadresistance 34, the other end of which is connected directly to the anodeof the SCR and to the nonpolarized condenser 42 the other side of whichis connected to the'primary P of the autotransformer 43. The other sideof the primary P of the autotap transformer 43 as well as the cathode ofthe silicon controlled rectifier are connected directly to line 2 as inthe previous circuit and the diode 44 is connected directly to the line6 to the filter 45 as in the previous circuit.

The pulse initiating circuit means in FIG. 8 is maintained by the neontube 46 one side of which is connected to the gate G of the siliconcontrolled rectifier 4] and the other element of the neon tube 46isconnected directly to one side of the condenser 47 the other side ofwhich is connected by the line 48 to the movable arm of thepotentiometer 50 one end of which is connected directly to the cathodeof the diode 30 and the opposite end is connected to one side of thecondenser 38 the other side of which is connected to line 2. Thiscircuit means provides substantially the same low frequency in pulsatingdirect current voltage and is not capable of delivering much more thanone tenth of a milliamp. Thus both circuits are very safe yet are veryefi'ective in inducing a field to a current carrying a grid as well as aplastic or insulating material for the purpose of collecting any chargedparticles such as dust as previously described.

The autotap transformer shown both in FIGS. 7 and 8 at 43 is preferablyof the spark coil type as used in automobiles or other internalcombustion engines. This type of an autotype transformer has very littleiron merely being provided with an iron core with a limited return. Mostof the flux must travel through the air from one end of the core to theother. This type of autotransformer provides a very sharp spikedischarge. It is very narrow and in a way approaches infinity but it isnot infinite but really high in comparison with other form of thetransients produced on the wave passing through the transformer primary.

As shown in FIG. 9 is a reproduction of the character of spike waveprovided showing the transients.

FIG. 9 shows the spike waveform produced by each of these tow circuits.The frequency of this pulse if from 60 to 70 times per second. The curveshown at 51 is a tracing of the 60 cycle voltage to indicate the time.The curve 52 shows the waveform at the end of the coaxial shielded lead6 with no load thereon. The spike is evident at 53 and it dampens tozero with little or no action as indicated at 54.

The curve 55 is a result of attaching the lead 6 to the filter. Withthis filter load, this monocharging spike voltage produces a terrificseries of reflected surges between the filter and the charging condenser42. The first charging spike 56 is reflected back by the second negativepeak 57 followed by the surges 58, 59, 60 and 61 and the lower dampeningpeak 62 which is a high positive kick that is believed to shut off theSCR. It is believed that these active portions of the spike pulsesproduce the percent efficiency in collecting charge carrying particleswithout the use of a return circuit.

It should be noted that after the high peak surges there appears adampening configuration. This circuit produces a spherical field aroundthe filter which when in open air is very strong. Without a pickup coila neon light has strong intensity at l0 feet. With a pickup coil it willindicate this charge many feet away.

When the charged screen is placed in a duct of a furnace, the shieldingof the metal is effective in confining this field, yet it may bemeasured inside of the furnace indicating that the field is extensive ina grounded ductwork.

I claim: 1. The method of inducing precipitation of normally chargedairborne particles from an air medium passing through a collectorconsisting of a grid contained filter medium of essentially dielectricmaterial, comprising the steps of;

gating a controlled semiconductor rectifier with low frequency pulses toproduce low frequency direct current charging pulses,

charging a loop circuit including a condenser and the primary of anautotransformer to induce low frequency spike type high voltage pulsesof short duration from the autotransformer secondary,

and discharging the low frequency direct spike type high voltage pulsesthrough a diode to the grid contained filter medium of dielectricmaterial to produce thereon an inductive field.

2. The method of claim I characterized by the step of;

providing an iron core in said autotransformer to induce low impedanceto generate a spike type high voltage discharge having a sharp peakvoltage of narrow width and short duration.

3. The method of claim 1 characterized by the step of;

controlling the rate of discharge of said condenser into saidautotransformer primary coil to control the spike type voltage pulsescharacterized by having a sharp peak of low frequency thereby providinga physically nonhazardous collector.

4. An inductive field charging apparatus for inductively charging afilter medium for collecting normally charged airborne particles passingthrough said filter medium comprising pulse initiating circuit means tosupply a low frequency DC pulse and including a gate controlledrectifier having its gate connected to said pulse initiating circuitmeans, a high voltage low impedance discharge circuit connected acrossthe anode and cathode of said gate controlled rectifier and including acharging condenser and an autotap transformer, said autotransformerconnected through a high voltage rectifier to said filter medium toinductively provide a monocharge thereto.

5. The inductive field charging apparatus of claim 4 characterized inthat said autotransformer has a primary coil connected in series withsaid charging condenser and a secondary coil connected through said highvoltage rectifier to said filter medium to inductively provide amonocharge thereto.

6. The inductive field charging apparatus of claim 4 characterized by anautotransformer of the spark coil type to form the low impedance spikedischarge through said high voltage rectifier.

7. The inductive field charging apparatus of claim 4 characterized inthat said pulse initiating circuit means includes a field effecttransistor having one base connected to said controlled rectifier gate,and a diode connected through load resistance to said controlledrectifier anode and to the other base of said field effect transistor.

8. The inductive field charging apparatus of claim 4 characterized inthat said pulse initiating circuit means includes a diode connectedthrough load resistance to said controlled rectifier anode and to acoupling condenser connected to a neon tube the end of which isconnected to said controlled rectifier gate.

9. The inductive field charging apparatus of claim 4 characterized inthat said filter medium includes an air permeable dielectric filamentarymaterial.

10. The inductive field charging apparatus of claim 9 characterized by ametallic grid embedded in said dielectric filamentary material.

11. The inductive field charging apparatus of claim 9 characterized inthat said dielectric filamentary material is cellular foam ofpolyethylene ester.

12. The inductive field charging apparatus of claim 4 characterized inthat said filter medium includes an air permeable metallic filamentarymaterial.

13. The inductive field charging apparatus of claim 4 characterized inthat said filter medium includes a combination of air permeabledielectric and metallic filamentary materials.

2. The method of claim 1 characterized by the step of; providing an ironcore in said autotransformer to induce low impedance to generate a spiketype high voltage discharge having a sharp peak voltage of narrow widthand short duration.
 3. The method of claim 1 characterized by the stepof; controlling the rate of discharge of said condenser into saidautotransformer primary coil to control the spike type voltage pulsescharacterized by having a sharp peak of low frequency thereby providinga physically nonhazardous collector.
 4. An inductive field chargingapparatus for inductively charging a filter medium for collectingnormally charged airborne particles passing through said filter mediumcomprising pulse initiating circuit means to supply a low frequency DCpulse and including a gate controlled rectifier having its gateconnected to said pulse initiating circuit means, a high voltage lowimpedance discharge circuit connected across the anode and cathode ofsaid gate controlled rectifier and including a charging condenser and anautotap transformer, said autotransformer connected through a highvoltage rectifier to said filter medium to inductively provide amonocharge thereto.
 5. The inductive field charging apparatus of claim 4characterized in that said autotransformer has a primary coil connectedin series with said charging condenser and a secondary coil connectedthrough said high voltage rectifier to said filter medium to inductivelyprovide a monocharge thereto.
 6. The inductive field charging apparatusof claim 4 characterized by an autotransformer of the spark coil type toform the low impedance spike discharge through said high voltagerectifier.
 7. The inductive field charging apparatus of claim 4characterized in that said pulse initiating circuit means includes afield effect transistor having one base connected to said controlledrectifier gate, and a diode connected through load resistance to saidcontrolled rectifier anode and to the other base of said field effecttransistor.
 8. The inductive field charging apparatus of claim 4characterized in that said pulse initiating circuit means includes adiode connected through load resistance to said controlled rectifieranode and to a coupling condenser connected to a neon tube the end ofwhich is connected to said controlled rectifier gate.
 9. The inductivefield charging apparatus of claim 4 chAracterized in that said filtermedium includes an air permeable dielectric filamentary material. 10.The inductive field charging apparatus of claim 9 characterized by ametallic grid embedded in said dielectric filamentary material.
 11. Theinductive field charging apparatus of claim 9 characterized in that saiddielectric filamentary material is cellular foam of polyethylene ester.12. The inductive field charging apparatus of claim 4 characterized inthat said filter medium includes an air permeable metallic filamentarymaterial.
 13. The inductive field charging apparatus of claim 4characterized in that said filter medium includes a combination of airpermeable dielectric and metallic filamentary materials.